Alumina ceramic envelopes are of particular interest in connection with high pressure sodium vapor lamps which are now widely used for outdoor lighting applications on account of their high efficacy, generally in excess of 100 lumens per watt. Alumina ceramic is resistant to sodium at high temperatures and both high density polycrystalline alumina and monocrystalline alumina or synthetic sapphire are utilized for lamp envelopes. The lamp fill comprises sodium along with a rare gas to facilitate starting, and mercury for improved efficiency. The ends of the alumina tube are sealed by suitable closure members affording connection to thermionic electrodes. The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with a mogul screw base. The electrodes of the arc tube are connected to the terminals of the base, that is to shell and center contact, and the inter-envelope space is usually evacuated in order to conserve heat.
One design of end seal for an alumina ceramic arc tube which is described in U.S. Pat. No. 3,882,346--McVey, utilizes an alumina ceramic plug sealed in the end of the arc tube and having a central perforation through which is sealed a lead wire of ceramic-matching metal, that is, of metal which approximately matches the coefficient of expansion of the ceramic, preferably niobium or alternatively tantalum in the present case where ceramic is polycrystalline alumina. The sealing is effected through a glassy sealing composition which melts when the assembly is suitably heated and forms the ceramic-to-ceramic and ceramic-to-metal seal.
At sealing, the assembly is heated until the sealing composition or frit is molten and is drawn by capillary action into the ring-like gap between arc tube and ceramic plug and into the inlead hole through the plug. A problem which has been encountered with this seal design is unevenness in the distribution of sealing frit as between the peripheral seal of arc tube to plug and the central seal of plug to inlead which may cause weak seals and leakers. The object of the invention is to remedy this problem.